1. Field of the Invention
The invention relates to a reciprocating pump for a cryogenic fluid comprising a pump cylinder made of a material with low thermal expansion, a piston displaceable in the pump cylinder, and piston rings made of a self-lubricating material with a larger coefficient of thermal expansion than the material of the pump cylinder held on the circumferential surface of the piston.
2. Description of the Prior Art
In reciprocating pumps for cryogenic fluids, for example, liquid nitrogen and liquid hydrogen, a number of problems arise due to the boiling state of the cryogenic fluids, their low temperatures and their low kinematic viscosity: The low temperatures limit the choice of materials to a considerable degree. Shrinkage problems occur, in particular, in the pairing of piston and cylinder. Use of additive lubricants is not possible. Owing to the low kinematic viscosity of the fluids to be pumped one is dependent on self-lubricating surfaces of piston and cylinder. Usually, sealing is effected by piston rings on the pistons with self-lubricating properties, for example, piston rings made of PTFE, PTFE-carbon, PTFE-graphite or PTFE-bronze. Pumps of this kind are known, for example, from U.S. Pat. Nos. 4,156,584 and 4,396,362 and also from the article by C. F. Gottzmann, "High-Pressure Liquid-Hydrogen and -Helium Pumps", AICE, Advances in Cryogenic Engineering, Volume 5, 1960, pp. 289-98.
With self-lubricating piston rings made of PTFE-graphite, PTFE-carbon or similar substances, good self-lubricating properties are obtained with respect to steel. However, the high thermal expansion coefficient of these piston rings in relation to the pump cylinder material, on the one hand, and to the piston material, on the other hand, is disadvantageous. When cooled from ambient temperature to 77 K., the thermal expansivity of PTFE is six to seven times higher than in high-grade steel and almost forty times higher than in Fe Ni 36 steel. The radial shrinking of the PTFE piston rings is, therefore, critical.
With slotted piston rings, the shrinkage can be compensated by spring pretensioning by means of beryllium copper springs, but the leak through the slot and the high manufacturing expenditure are disadvantageous.
With unslotted PTFE piston rings, the gap between piston and cylinder which increases in size during cooling-down can be reduced by a combination of several measures:
1. The piston ring thickness is reduced as far as technically possible in order to reduce the absolute shrinkage;
2. By shrink-fitting the ring on an Fe Ni 36 piston, the internal diameter of the piston ring remains practically constant during cooling-down so that the lateral contraction is the only decisive factor;
3. By using austenitic steels which are tough at low temperatures as cylinder material, the gap is finally reduced to the difference between the lateral contraction of the PTFE and the shrinkage of the cylinder made of tough austenitic low-temperature steel. The sealing achieved in this way is still insufficient for high-pressure pumps (pressure increase &gt;10 bar).
Departing from a reciprocating pump of the generic kind, the object underlying the invention is to achieve substantially temperature-independent sealing between piston rings and pump cylinder although the thermal expansion coefficients of the piston ring material and the cylinder material are different.
This object is attained in accordance with the invention in a reciprocating pump of the kind described at the outset by the piston having a core made of a material with relatively large thermal expansivity surrounded by a sleeve made of a material with low thermal expansion, by the core protruding on both sides from the sleeve and having expanding regions conically increasing towards its free ends and by the piston rings surrounding the core in the expanding regions and being supported against the end faces of the sleeve.
Owing to this design, the axial contraction of the core of the piston during cooling-down is greater than that of the surrounding sleeve. Hence during cooling-down the piston rings on the conically expanding regions of the core are axially displaced into regions of larger diameter. This results in expansion of the piston rings. In this case, the dimensions may be selected such that this expansion of the piston rings by the expanding regions of the core compensates the shrinkage of the piston rings to such an extent that the resulting shrinkage of the piston rings corresponds to the shrinkage of the pump cylinder dimensions.
In a preferred embodiment, the piston rings directly abut the conically expanding regions of the core, and, therefore, undergo axial displacement on the core during cooling-down.
The core may consist of two components joined together within the sleeve. This facilitates assembly of the piston.
In another preferred embodiment, the expanding regions of the core are surrounded by a bearing ring divided up into segments by radial cuts to enable radial expansion of the bearing ring when axially displaced on the conically expanding region. The bearing ring is supported against the end face of the sleeve and the piston ring surrounds the bearing ring and is held on it. In this embodiment, it is the bearing ring that first undergoes expansion during cooling-down and it then transfers its expansion to the piston ring surrounding it.
In this case, it is expedient for a conical surface of the bearing ring to abut the conical surface of the expanding region of the core, with the conicity of both parts being substantially identical.
The expanding region of the core may be positioned on and releasable from the core at least at one of its ends. This also facilitates piston assembly.
The expanding regions preferably comprise axially protruding flanges acting as axial stop for the piston ring.
The following description of preferred embodiments serves in conjunction with the appended drawings to explain the invention in greater detail. In the drawings: